Some good thought out arguments here.
I'd add that Glasner is a difficult Manager to follow, especially when his players are being sold. I think one of the mistakes made was choosing a manager that was expected to continue in Glasners mould, rather than allowing/choosing a Manager and give him space to create his team.
The problem is that Glasner's success was built on his ability to find and instruct a small core of players and get them to play exactly how they want them to play. We weren't exceptional but we were effective and when it worked it could have been mistaken for free flowing attacking football, where in essence we were solid, and Broke with speed and precision. He avoided changing the lineup too much as he knew his system required particular qualities and players he trusted to deliver them.
He was widely revered as the only man who could make a 3 man defence work, but it took a little time and some work to get there and with his players slowly leaving whilst he was still here we saw how difficult the PL became; he won the conference as the quality is so inferior.
It feels as though Sage was picked as he was a 3-at-the-back man and we've hold him that that's how we play, and it's how we'd directed him to continue; expected to play the same formation which is difficult to implement with different players. It was always going to be a big task.
I think Sage is a decent manager, but was the wrong choice post-Glasner. OG was given time, and an unpressured environment, with no european commitments or expectations with which to build his team. Sage would (as would most) have more success in that environment.